This invention relates to a method and apparatus for improving detection of the presence of the wheels of a train in a predetermined stretch of railroad track by ensuring that an effective shunt is maintained across the rails and, in particular, to the utilization of a normally inactive, high-voltage pulse generator which is connected to the rails and enabled when a train is present, and an associated receiver that disables the generator when train wheels are no longer present.
The shunt effect caused by the metallic wheels and axles of a railroad train is an important characteristic which is relied upon in the design and practical utilization of control systems for railroads. Since the shunt is a short circuit across the rails where a train is located, the change in track impedance which results when a train approaches, as seen, for example, from a grade crossing may be used to detect the motion of the train at a distance from the crossing and activate appropriate highway-rail grade crossing warning devices such as flashing lights and/or gates to warn vehicular traffic when the train enters a designated area on either side of the crossing. When the train reaches the grade crossing, typically a separate island detector then assumes a presence detection function to continue the warning system in operation during the time that the train is physically present in the crossing. Therefore, the effective and consistent establishment of a shunt across the rails by the wheels and axles of the train is vital to the operation of the warning system.
In recent years, a number of developments intended to improve rolling efficiency have made it increasingly difficult to rely upon a consistent shunt. Changes in railhead contours caused by milling the edge of the rail (reducing the normal scrubbing between the wheel and the railhead), wheel oiling to reduce rolling friction, and articulated cars with reduced axle counts all impair the establishment of a consistent shunt. Also, oxide buildup (rust) on the top of the rail reduces normal rail/wheel contact. Therefore, both physical changes and the presence of contaminates provide a potential of reducing electrical contact to the point where grade crossing warning systems, which are activated by this contact, may at times be inappropriately deactivated even though a train still occupies the island section of the highway-rail grade crossing warning circuit.
It has been proposed heretofore to apply an additional 60 Hz alternating current to the track for the purpose of breaking through films and oxides which may be present in order to improve the electrical contact between the train wheels and the rails over which they travel. However, experience indicates that such a low frequency signal applied at an energy level sufficient to overcome the insulating effects of the contaminants is subject to being shunted by train wheels outside of, and distant to, the island section of track where the wetting benefit is required.
Also, the application of a wetting current to the track has been proposed as disclosed in U.S. Pat. No. 5,170,970, issued Dec. 15, 1992. The wetting current is provided by a high-frequency power oscillator that is activated by an existing island circuit and runs continuously while a train is present in the island. The frequency (10 KHz or higher) provides suitable inductive isolation but the frequency generated can create interference problems with other control equipment located at the crossing.